The present invention is directed to an improved excimer lamp.
In recent years, many industrial processes have been developed which use ultraviolet radiation to treat material. In some such processes, the material has a photocurable coating thereon, and the ultraviolet radiation "cures" the coating by a chemical reaction. Such photocurable coatings may for example be clear or pigmented, and are applied to a variety of objects including flat substrates and curved objects such as cans. Photocurable coatings are also used in practicing semiconductor photolithography, which is a process which is used in the manufacture of integrated circuits.
The ultraviolet lamp which is used to irradiate photocurable coatings typically utilizes a bulb fill which contains mercury, with an additive or additives sometimes being present to add emphasis to a particular region or regions of the spectrum. Thus, the spectrum of the light which is emitted by the lamp is the spectrum of the element mercury, or that of mercury plus a particular additive.
The spectrum of mercury as generated by such lamps has radiation present throughout the fairly broad spectral band of 200-400 nanometers. Since the radiation is distributed throughout the entire band, the efficiency of the lamp in any particular narrower part of the entire band is relatively low.
For some applications, it would be desirable to have most of the radiation output of the lamp within a narrower band. By way of example, some photocurable materials are much more responsive to ultraviolet radiation within the relatively narrow 250-300 nanometer band than to radiation in other parts of the ultraviolet spectrum. Such a material would be more rapidly cured by a lamp which has most of its output concentrated within the 250-300 nanometer band.
In recent years, discharge devices which emit excimer radiation have become known. Excimers are unstable excited complexes of molecules that under normal conditions possess an unbound or weakly bound ground state, and thus are not known from classical physics. That is, the excimer complexes exist only in the excited state. The excimer complexes disintegrate within less than a microsecond, and during their decay give off their binding energy in the form of radiation in a narrow band.
While the most common excimer devices are lasers, recently a microwave powered excimer lamp has been disclosed. In the article entitled "New High-Efficiency Quasi-Continuous Operation of A KrF (B.fwdarw.X) Excimer Lamp Excited by Microwave Discharge" by Kumagai and Obara, Applied Physics letters, Vol. 54, No. 26, Jun. 26, 1989, pp. 2619-2621, a lamp which emits radiation in a very narrow band of several nanometers about 248 nm is discussed. The lamp utilizes a fill having small amounts of fluorine and krypton in a buffer gas mixture of helium and neon. The article teaches that it is required to operate the lamp at a low pressure to achieve suitable efficiency, with an efficiency of 12.1% being attained at a desired total pressure of 50 torr and a halogen pressure of 1 torr. This efficiency may not be high enough for certain applications.